Sniper bolt warping when rotated in pose mode
If a vertex is assigned to two or more bones, its movements will be conditioned by the position of all the bones involved, while if it's assigned to one bone only (even with a small amount of weight) it will follow the movements of that bone exactly.

The "correct" way to rig, in your case, could be to have a bone controlling all the gun except the bolt vertices, and a second bone - child of the "main" bone - controlling only the bolt vertices. 

In this way you can avoid any deformation of the objects.

Don't use automatic weighting, use "with empty gorups" option instead, then select all the bolt, select the bolt bone's vertex group and click "Assign" button underneath the vertex group list.

Do the same for the main bone (which has to be the parent of the bolt bone), selecting all vertices except the bolt.